The Martian Sky Pulses in Ultraviolet Every Night

There’s a surprising phenomenon taking place in Mars’ atmosphere: during the spring and fall seasons on the Red Planet, large areas of the sky pulse in ultraviolet light, exactly three times every night.  

“The ultraviolet glow … is as bright in the ultraviolet as Earth’s northern lights,” said Zac Milby, of the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP). “Unfortunately, the composition of Mars’ atmosphere means that these bright spots emit no light at visible wavelengths that would allow them to be seen by future Mars astronauts.”

This is too bad, Milby said, as it would be quite a sight: the bright patches would intensify overhead every night after sunset, and drift across the sky at 300 kilometers per hour (about 180 miles per hour).

This finding comes from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, using the Imaging Ultraviolet Spectrograph (IUVS) on board the spacecraft. The instrument made observations of nitric oxide ultraviolet nightglow over two consecutive Martian years. The new data revealed unexpected ultraviolet waves and spirals over the winter poles, and an unusually bright spot on Mars’ nightside near the equator.

The ultraviolet emissions come from the recombination of nitrogen and oxygen atoms in Mars’ nightside mesosphere, the science team said in their paper, which was published in the Journal of Geophysical Research, Space Physics. “Emissions are brightest around the winter poles, with equatorial regions brightening around the equinoxes.”

The brightenings occur where downward, vertical winds carry atoms of oxygen and nitrogen down to higher pressures and densities where chemical reactions are sped up, and these reactions emit ultraviolet light.

The nightglow was measured at about 70 kilometers (approximately 40 miles) in altitude, with the brightest spot about a thousand kilometers (approximately 600 miles) across.

The diagram explains the cause of Mars’ glowing nightside atmosphere. On Mars’ dayside, molecules are torn apart by energetic solar photons. Global circulation patterns carry the atomic fragments to the nightside, where downward winds increase the reaction rate for the atoms to reform molecules. The downwards winds occur near the poles at some seasons and in the equatorial regions at others. The new molecules hold extra energy which they emit as ultraviolet light. Credits: NASA/MAVEN/Goddard Space Flight Center/CU/LASP

Why does this happen exactly three times every night? Nick Schneider, also from LASP and lead author of the paper, said it is because there are three locations on Mars where downward winds are enhanced. He also explained that this ‘factor of three’ is not unusual.

“In fact, many atmospheric patterns on Mars occur three times per day,” he said in an email to Universe Today. “This is turns out to be a natural response frequency to an atmosphere of this size influenced by sunlight, rotation, and topography (like mountains). Many objects, from musical instruments to dropped silverware have natural frequencies for vibration, and larger objects have lower frequencies. For the Mars atmosphere, three time per day is one of the most important frequencies. Atmospheric temperatures and pressures commonly vary three times per day, and these drive changes in downward winds.”

This is an image of the ultraviolet “nightglow” in the Martian atmosphere over the south pole. Green and white false colors represent the intensity of ultraviolet light, with white being the brightest. The nightglow was measured at about 70 kilometers (approximately 40 miles) altitude by the Imaging UltraViolet Spectrograph instrument on NASA’s MAVEN spacecraft. A simulated view of the Mars globe is added digitally for context, and the faint white area in the center of the image is the polar ice cap. The image shows an unexpectedly bright glowing spiral in Mars’ nightside atmosphere. The cause of the spiral pattern is unknown.
Credits: NASA/MAVEN/Goddard Space Flight Center/CU/LASP

The team wrote in their paper that the new finding “reveals the effects of global-scale winds and waves in the upper atmosphere from the change of wind patterns with the seasons, with circulation patterns transitioning from a single cross-equatorial cell operating during solstice periods to more symmetric equator-to-poles circulation around the equinoxes.”

“MAVEN’s images offer our first global insights into atmospheric motions in Mars’ middle atmosphere, a critical region where air currents carry gases between the lowest and highest layers,” said Schneider.

MAVEN’s findings confirm the nightglow observations first made by the SPICAM instrument on the European Space Agency’s Mars Express spacecraft. The next-generation instrument on MAVEN was better able to repeatedly map out the glow, finding patterns and periodic behaviors.

To follow up on these observations, the team plans to use the spacecraft to look at the nightglow in an oblique or “sideways” view, instead of the nadir, or straight down from above, the usual view of orbiting spacecraft. The oblique view can provide a three-dimensional-type view, which can hopefully add to a more accurate understanding of the vertical winds and seasonal changes.

Further reading:
NASA press release
Paper: Imaging of Martian Circulation Patterns and Atmospheric Tides Through MAVEN/IUVS Nightglow Observations